The present invention relates to apparatus for use in measuring and controlling fluid flow across a boundary. More particularly, the present invention relates to an apparatus useful in balancing or controlling air flows between different regions separated by partitions having air flow passageways. The invention finds advantageous application, for example, in the balancing of air flow into or out of spraybooths used in commercial painting operations.
Production painting operations for decorative or protective coatings are commonly conducted in spraybooths. The spraybooth is a chamber designed so that air supplied to one side or the top of the booth flows smoothly over the goods to be painted and is exhausted out the opposite side or bottom of the enclosure. This air flow carries excess paint spray away from the goods so that it cannot cause defects, and away from the equipment in the spraybooth to avoid fouling it with accumulated paint. In addition, a spraybooth will commonly have openings called "silhouettes" at either end to permit goods to be conveyed through the booth without a need to open or close doors as production proceeds. It is common to cause air to flow through such silhouettes in a chosen direction at a controlled velocity by carefully adjusting the supply and exhaust air volumes to the spraybooth. The air flow through the silhouette may serve to contain overspray inside the spraybooth if flowing in; or may serve to keep dirt or contamination outside the spraybooth if flowing out. As spraybooths are commonly divided into many sections for different process steps separated by silhouettes, the air balance of a spraybooth may be very complex.
Precise control over the static pressure inside a spraybooth enclosure with respect to the area around it is vital to the control of the painting process. If the booth is positive to the surrounding area, paint overspray will be blown out of the booth enclosure. If the booth is too negative, dirty air may be pulled into the booth and contaminate the product. In most cases, static pressure control is used to maintain or control flow velocities through silhouettes or other openings in the booth enclosure. However, pressure imbalances may create undesirable air flow patterns inside the booth which adversely affect product quality, system transfer efficiency, or worker safety. Also, since booths are commonly connected to other equipment, such as ovens and sanding operations, by enclosed tunnels, an imbalance in the spraybooth pressure may cause undesirable air balance changes in other equipment located distantly but connected to the booth by the tunnel system or other enclosures.
Conventionally, spraybooth air balance has been controlled manually or with static pressure controllers. Manual control is inefficient because the systems are large and complex and balance adjustments are required frequently. Automatic static pressure control has also proven ineffective because the desired control point is at a very low pressure level, undetectable by most commercially practical instruments. In addition, momentary pressure surges from such events as doors opening or initiation or termination of equipment operation may send the few available sensors or controllers of adequate sensitivity into off-scale readings causing difficulties with control. Any sensor attached to a spraybooth is also likely to become fouled with paint overspray in the normal course of production or maintenance. Few sensors of adequate sensitivity can survive an encounter with wet paint and maintain their function or accuracy. Due to these problems, past attempts at automatic air balance control in painting systems have generally been inadequate.
While past attempts at automatic control have relied on pressure controllers or sensors, the actual objective is to accurately control and stabilize the air velocities passing through openings in the spraying enclosure or related equipment. Typically, the desired velocities are on the order of 25-150 feet per minute. Given the large size of the booth openings, the 0.0001 required to generate these velocities are less than 0.001 inches of water column, often as low as 0.001 inches of water column. Such low velocities may be easily measured with hot wire anemometers and other low velocity instruments. However, such instruments have been previously considered impractical because they are very sensitive to fouling by paint overspray, which is inexorably drawn through the instrument in the course of normal operation. In addition, most such instruments are not bi-directional devices. They read in only one direction, or read only absolute velocities without regard to the direction of the flow. It is also desireable and often necessary that the instrument be intrinsically safe; that is, it must employ components and wiring not capable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of its environmental atmospheric mixture even when that mixture is composed of its most easily ignitable concentration. In paint spraybooth applications, an intrinsically safe instrument must be rated for use in a Class I, Division I, Group D explosive environment.
Thus, a need exists for a useful instrument adaptable to paint finishing systems and which is capable of balancing air flows under the following conditions: (1) read from 20-150 fpm reliably and repeatably; (2) be resistant to momentary pressure pulses; (3) indicate the direction of flow; (4) resist paint overspray or other contamination; and (5) be intrinsically safe.